Chapter 17

A Typical Day in the Life of Your Brain!
Perceiving, recognizing, recording, pondering, understanding, responding

The basic numbers/facts about the brain provided in this superficial description of the brain and the cerebral cortex largely come from Estimates of total cell numbers (both neurons and glial cells) have been ‘corrected’ in contemporary studies using more accurate cell-counting methods. (See Herculano-Houzel S (2012) The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost. PNAS 109:10661; and as background (2012) Neuronal rules for primate brains: the primate advantage. Prog Brain Res 195:325.) The division of the cortex into about a hundred functional areas began with the examination of the detailed cellular anatomy of the cortex by Brodmann, von Economo, Rose and others, where subtle details in “cytoarchitectonics” led to the identification of >40 identified areas. It should be noted that almost all areas have the same brain cells, and that for any given vertical cortical column of any given width, the same numbers of cells. On the other hand, each area is differentiated by often-subtle anatomical details because those details are changed, via plasticity, as a result of how each area is engaged as a function of its area-specific inputs, and the statistically defined, variably-correlated information that is fed to them.

The role of inputs in defining functional and anatomical details that distinguish cortical areas one from another was elegantly demonstrated, for example, by studies of Mriganka Sur and colleagues, who re-routed inputs from the visual thalamus into the auditory cortex, simultaneously depriving auditory cortex of its natural primary (auditory) input source. Visual inputs—with their point-to-point representational topography, eye-specific sources and center-surround receptive field structures all strikingly different from auditory inputs—generated a cortex that had the specific functional features of a primary visual cortex. Moreover, animals appeared to SEE when this novel now-visual (normally, a hearing) cortical area was activated. See, e.g., Horng SH, Sur M (2006) Visual activity and cortical rewiring: activity-dependent plasticity of cortical networks. Prog Brain Res 157:3.

Over the past several decades, scientists (primarily via human brain imaging studies strongly influenced by parallel studies in macaque monkeys) have subdivided many of the original approximately 40 areas to expand the number of distinctly defined functional zones to roughly 100 (distinguished because they are fed by different sources of inputs, have different intrinsic functional roles, and have different destinations for their outputs). These are the “very big (cortical) computers” described in the text.

The appreciation that the basic functional unit of the cortex is the “minicolumn” (my “350 million micro-processors”) arose in large part from the studies of my professor Vernon Mountcastle, and from David Hubel and Torsten Weisel. See Mountcastle VB (1997) The columnar organization of the neocortex. Brain 120:701. Also see Buxhoeveden DP, Casanova MF (2002) The minicolumn hypothesis in neuroscience. Brain 125:935. Minicolumns have been described as physical cortical entities; mini-columns are smaller in primates than in other mammals (we have more of them, proportionally—which confers a primate but not a human-over-other-primate advantage). (See Buxhoeveden DP, 2012, Minicolumn size in the human cortex. Prog Brain Res195:219.) At the same time, studies in my own lab have shown that functionally-defined mini-columns (which are easy to reconstruct via recording in most brain areas) in non-primates or primates can be modified (reduced OR increased in volume) over a total range of several hundred-fold to several thousand-fold by specific, simple forms of brain training. See, e.g., Merzenich MM, DeCharms RC (1996) Neural representations, experience and change. In: Mind-Brain Continuum, Llinas R, Churchland P, eds. MIT Press.

Once George woke up, see annotation for other Chapters for notes on specific issues or mechanism described to be in play, at different moments on his brain’s “typical day”.